REPRODUCTION IN LABORATORY COLONIES OF CLETHRIONOMYS GAPPERI AND MICROTUS-PENNSYLVANICUS - A COMPARISON BETWEEN SPECIES AND AMONG POPULATIONS

Reproduction in laboratory colonies of Clethrionomys gapperi and Micro tus pennsylvanicus was examined by comparing six populations (three pe r species) to test the general hypothesis that populations subject to the lowest temperatures and the shortest breeding seasons would be at the ''fast'' end of the ''fast-slow'' continuum. All colonies were der ived from three sites in western Canada from females that were insemin ated in the wild. Postpartum mass, mass of adult females during lactat ion, litter size, litter and neonate masses at birth, litter mass at w eaning, age when the eyes opened, and age at weaning as well as two va riables describing the energetics of reproduction were examined betwee n species and among populations within species. Three indices of physi ological reproductive effort were also compared. Only neonate mass, ag e when eyes were open, and one index of reproductive effort differed b etween species. In C. gapperi, litter mass, litter size, age when the eyes opened, age at weaning, and one index of reproductive effort diff ered among populations. In M. pennsylvanicus, postpartum, litter (at b irth and weaning), and neonate mass, age when eyes open, age at weanin g, and maintenance costs during lactation were different among populat ions. In both species, some differences could be attributed to samplin g biases, while others were simply a consequence of differences in mat ernal mass. In both species, the maximum difference in the age at wean ing and the age when the eyes opened was less than 2 days. Differences among populations did not appear to be related to meteorological cond itions, and populations in either species could not be ranked on a con tinuum.